<p dir="ltr">Aquatic ecosystems and their inhabitants are no strangers to stressors. Natural stressors like disease, competition, and predation have a near constant presence in these environments and are often accompanied by human-induced stressors like climate change and chemical contaminants. Chemical contaminants like pesticides are often found in aquatic systems located near agriculture and can have detrimental effects on wildlife. Although natural stressors and pesticides often occur at the same time, their combined interactions still require further investigation to understand, as some pesticides, like fungicides, are frequently under researched. Additionally, fungicides also are lacking in research pertaining to combined chemical stressors. Studying the interactive effects of combined stressors, whether natural or human-induced, is crucial for applying laboratory findings to natural environments.</p><p dir="ltr">Here, I conducted an outdoor mesocosm experiment and multiple laboratory experiments to broadly assess the interactive effects of the fungicide chlorothalonil. More specifically, I explored 1) the interactive effects of the herbicide atrazine and chlorothalonil on an aquatic community, 2) the interaction between chlorothalonil and predator-induced stress of three tadpole species, and 3) the impacts of chlorothalonil on amphibian predator-prey dynamics using tadpoles and larval salamanders. First, to test my hypothesis that atrazine and chlorothalonil will have combined interactions that negatively impact an aquatic community, I conducted a mesocosm experiment where I exposed an aquatic community to atrazine, chlorothalonil, and the combination of the two pesticides. I found that the two pesticides do hold the potential to interact in certain cases, but their main effects alone are often just as damaging, especially for chlorothalonil where an environmentally relevant concentration caused near total morality for bullfrog tadpoles.</p><p dir="ltr">Next, I conducted a laboratory experiment with tadpoles and caged predators exposed to a sublethal concentration of chlorothalonil to test my hypothesis that the combination of predator-induced stress and chlorothalonil will decrease tadpole survival and alter tadpole behavior. I found that chlorothalonil alters tadpole behavior by significantly reducing activity levels in all three study species. However, no evidence was found for the interaction of the two stressors decreasing survival.</p><p dir="ltr">Lastly, to test my hypothesis that the toxicity of chlorothalonil will reduce tadpole behavioral responses and thereby increase their predation risk, I conducted a laboratory experiment with free-swimming tadpoles as prey and larval tiger salamanders as predators, exposed to different combinations of chlorothalonil exposure for the prey or predator. I found no meaningful differences in survival curves for the different combinations of prey and predator, but leopard frog tadpole final survival was lower in the higher chlorothalonil dose treatment due to predation.</p><p dir="ltr">Overall, these studies have assisted in filling research gaps on fungicides and their impact on predator-prey dynamics and aquatic communities. This work provided insights into the direct impacts of the herbicide atrazine and the fungicide chlorothalonil on aquatic species, and their potential to interact with natural stressors, emphasizing the need to protect natural ecosystems from chemical stressors.</p>

<p> Per- and polyfluoroalkyl substances (PFAS) are a class of persistent environmental contaminants that have become ubiquitous, resulting in widespread exposure among humans and wildlife. Amphibians are regularly exposed in the field, making them susceptible to sublethal effects of PFAS exposure. In amphibians exposed to PFAS, deleterious effects have been observed, including reduction in body condition measured using the scaled mass index (SMI) and degraded hepatic condition, among others. PFAS may dysregulate lipid metabolism by altering signaling cascades regulated by peroxisome proliferator activated receptors (PPAR), but whether changes in energy stores can explain changes in amphibian SMI and/or hepatic condition remain underexplored. Since lipids are a critical energy reserve for anurans, understanding whether lipid metabolism is being perturbed is critical. The central objective of this thesis was to investigate the effect of PFAS on lipid homeostasis in <em>Xenopus laevis </em>tadpoles within the context of a PPAR mechanism of action (MOA), considering apical, molecular, and lipidomic endpoints. I conducted three studies: (a) a study to characterize SMI and the relative expression of the hepatic xPPARα/β/γ during metamorphosis, (b) a pharmaceutical exposure to assess the <em>in vivo</em> effects of xPPARα/β/γ agonism on hepatic gene expression for select downstream targets (<em>apoa5, fabp1, acox1,​ pck1</em>), and (c) a chronic PFAS exposure to investigate the effects of environmentally relevant concentrations (PFOS, PFHxS, PFOA, PFHxA at 0.5 ppb; binary mixture of PFOS:PFHxS at 1 ppb) on lipid homeostasis through apical endpoints (mass, snout vent length, SMI, hepatic condition), relative hepatic gene expression, and Multiple Reaction Monitoring (MRM) profiling of the hepatic lipidome for changes in relative class abundance. In study (a), I identified SMI and hepatic expression of <em>xPPARα/β/γ</em> is dynamic during late metamorphosis, indicating the potential for heightened susceptibility. However, in study (b), pharmaceutical agonists had no effect on <em>X. laevis</em> at high doses. For study (c), I did not observe effects on a majority of apical endpoints, including SMI, but detected a significant sex-specific reduction in hepatic condition for male<em> X. laevis</em> tadpoles exposed to single-chemical perfluorosulfonic acid (PFSA) treatments. For gene expression, I observed a transient downregulation for apolipoprotein-V (<em>apoa5</em>) at Nieuwkoop and Faber (NF) stage 62 for <em>X. laevis</em> tadpoles exposed to single-chemical perfluorocarboxylic acid (PFCA) treatments. Lipid profiling detected transient dysregulation of predominantly membrane lipids in-response to short-chain PFAS treatments at NF 58. Overall, our findings indicate PFAS may exert toxicity during anuran metamorphosis through multiple mechanisms of action (MOA) with sex-specific and developmental-stage specific outcomes.</p>